This invention relates in general to an electroforming process and more specifically to a process for electroforming hollow articles having a small cross-sectional area.
The fabrication of hollow articles having a large cross-sectional area may be accomplished by an electroforming process. For example, electrically conductive, flexible, seamless belts for use in an electrostatographic apparatus can be fabricated by electrodepositing a metal onto a cylindrically shaped mandrel which is suspended in an electrolytic bath. The materials from which the mandrel and the electroformed belt are fabricated are selected to exhibit different coefficients of thermal expansion to permit removal of the belt from the mandrel upon cooling of the assembly. In one electroforming arrangement, the mandrel comprises a core cylinder formed of aluminum which is overcoated with a thin layer of chromium and is supported and rotated in a bath of nickel sulfamate. A thin, flexible, seamless band of nickel is electroformed by this arrangement. In the process for forming large hollow articles having a large cross-sectional area, it has been found that a diametric parting gap, i.e. the gap formed by the difference between the average inside electroformed belt diameter and the average mandrel diameter at the parting temperature, must be at least about 8 mils and preferably at least about 10-12 mils (or 0.04-0.06 percent of the diameter of the mandrel) for reliable and rapid separation of the belt from the mandrel. For example, at a parting gap of about 6 mils, high incidence of both belt and mandrel damage are encountered due to inability to effect separation of the belt from the mandrel.
The parting gap is dependent upon the macro stress in the belt, the difference in linear coefficients of thermal expansion between the electroformed nickel and mandrel material and the difference between the plating and parting temperatures, in the following manner.
Parting Gap=Delta T (Alpha.sub.M -Alpha.sub.Ni)D-S.D/E.sub.Ni Greater or Equal 0.008 in.
wherein D is the diameter of the mandrel (inches) at plating temperature; S is the internal stress in the belt (psi) E.sub.Ni is Young's modulus for nickel; Delta T is the difference between the plating temperature and the parting temperature and Alpha.sub.M -Alpha.sub.Ni are the linear coefficients of thermal expansion between the mandrel material (M) and the electroformed nickel (Ni).
One process for electroforming nickel onto a mandrel is described in U.S. Pat. No. 3,844,906 to R. E. Bailey et al. More specifically, the process involves establishing an electroforming zone comprising a nickel anode and a cathode comprising a support mandrel, the anode and cathode being separated by a nickel sulfamate solution maintained at a temperature of from about 140.degree. F. to 150.degree. F. and having a current density therein ranging from about 200 to 500 amps/ft.sup.2, imparting sufficient agitation to the solution to continuously expose the cathode to fresh solution, maintaining this solution within the zone at a stable equilibrium composition comprising:
______________________________________ Total Nickel 12.0 to 15.0 oz/gal Halide as NiX.sub.2.6H.sub.2 O 0.11 to 0.23 moles/gal H.sub.3 BO.sub.3 4.5 to 6.0 oz/gal ______________________________________
electrolytically removing metallic and organic impurities from the solution upon egress thereof from the electroforming zone, continuously charging to the solution about 1.0 to 2.0.times.10.sup.-4 moles of a stress reducing agent per mole of nickel electrolytically deposited from the solution, passing the solution through a filtering zone to remove any solid impurities therefrom, cooling the solution sufficiently to maintain the temperature within the electroforming zone upon recycle thereto at about 140.degree. F. to 160.degree. F. at the current density in the electroforming zone, and recycling the solution to the electroforming zone.
The thin flexible endless nickel belt formed by this electrolytic process is recovered by cooling the nickel coated mandrel to effect the parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion.
As apparent in the disclosure of U.S. Pat. No. 3,844,906, a difference in the thermal coefficients of expansion of the electroformed article and mandrel is a vital factor in the electroforming process described therein for obtaining a sufficient parting gap to remove an electroformed article from the mandrel. For nickel belts having a diameter of about 21 inches, the difference in thermal coefficient of expansion between the electroformed article and the mandrel contributes about 60 percent to about 70 percent of the principal factors contributing to the formation of an adequate parting gap. The remaining 40 percent to 25 percent factor for an adequate parting gap for a belt of this size produced by the process of U.S. Pat. No. 3,844,906 is the internal stress (compressive) in the metal. This internal stress is controlled by stress enhancers or reducers and is independent of any differences in temperature. Typically, stress reducers are added to maintain a compressive condition. Sodium saccharin is added to the process described in U.S. Pat. No. 3,844,906 to control internal stress. However, differences in the thermal coefficients of expansion of the electroformed article and the mandrel contribute very little to the parting gap for hollow electroformed articles having a small cross-sectional area and stress reducers need not be used. Thus, for hollow electroformed articles having a relatively large cross-sectional area, the difference in the thermal coefficient of expansion of the electroformed article and the mandrel are significant and determine, for example, whether heating or cooling is necessary to secure the necessary parting gap. More specifically, nickel has a thermal coefficient of expansion of 8.3.times.10.sup.-6 in/in/.degree.F., aluminum has a thermal coefficient of expansion of 13.times.10.sup.-6 in/in/.degree.F., and stainless steel has a thermal coefficient of expansion of 8.times.10.sup.-6 in/in/.degree.F. When large diameter nickel articles are electroformed on mandrels of aluminum or aluminum coated with chromium, parting is assisted primarily by the difference in the thermal coefficients of expansion of the electroformed article and the mandrel when the assembly is cooled. However, when large diameter aluminum articles are electroformed on a stainless steel or nickel mandrel, heat must be applied to the assembly to assist parting. When large diameter nickel articles are electroformed on a stainless mandrel, the thermal coefficient of expansion of nickel is only slightly higher than that of stainless steel so that neither heating nor cooling of the assembly assists in removing the electroformed article from the mandrel.
However, when metal articles are fabricated by electroforming on mandrels having a small cross-sectional area, difficulties have been experienced in removing the electroforming article from the mandrel. For example, when the chromium coated aluminum mandrel described in U.S. Pat. No. 3,844,906 is fabricated into electroforming mandrels having very small diameters of less than about 1 inch, metal articles electroformed on these very small diameter mandrels are extremely difficult or even impossible to remove from the mandrel. Attempts to remove the electroformed article can result in destruction or damage to the mandrel or the electroformed article, e.g. due to bending, scratching or denting. Although aluminum has a relatively high thermal coefficient of expansion, such expansion is normally not great enough to impart a sufficient parting gap to allow removal of hollow electroformed articles from mandrels having a small cross-sectional area. Harder materials having high strength such as stainless steel have a significantly lower thermal coefficient of expansion than aluminum and would render even more difficult the removal of hollow small diameter electroformed articles therefrom. Although removal of an electroformed article depends to some extent on the characteristics of the mandrel such as smoothness, strength, length and coefficient of expansion, the diameter or cross-sectional area of the mandrel becomes the determining factor as to whether an electroformed article may be removed as the diameter or cross-sectional area of the mandrel becomes smaller and smaller. For large nickel belts, having a diameter of about 21 inches, the parting gap is about between 10 and 12 mils. For nickel cylinders having a diameter of about 3.3 inches, the parting gap is between about 2 and about 4 mils. As the diameter becomes smaller, for example about 1.75 inches, the parting gap drops to between about 1 and about 2 mils and the parting gap for a 1 inch diameter cylinder is about 1/2 mil. All of the above pertain to a nickel sleeve on a mandrel having a hollow aluminum core and chromium outer coating. Since the parting gap must be at least about 8 and preferably between about 10 to 12 mils and since a difference between the thermal coefficients of expansion of the mandrel and electroformed article are both nececessary for reliable and rapid separation of the mandrel as indicated in U.S. Pat. No. 3,844,906, it is readily evident that small diameter mandrels, even those having a high thermal coefficient of expansion, fail to function as suitable mandrels for electroformed articles having a small diameter or small cross-sectional area.
Accordingly, it is an object of this invention to provide an electroforming process which electroforms hollow articles having a small cross-sectional area.
It is another object of this invention to provide a process for electroforming hollow articles having a small cross-sectional area that are readily removable from mandrels regardless of whether a difference exists in the coefficients of expansion of the electroformed article material and the mandrel material.
It is still another object of this invention to provide a process for electroforming articles on mandrels having a thermal coefficient of expansion lower than the thermal coefficient of expansion of the electroformed article material.
It is another object of this invention to provide a process for electroforming an article on a mandrel in which the electroformed article has a thermal coefficient of expansion substantially equal to the thermal coefficient of expansion of the mandrel.
These as well as other objects are accomplished by the present invention by providing an electroforming process comprising providing a core mandrel having an electrically conductive, abhesive outer surface, a coefficient of expansion of at least about 8.times.10.sup.-6 in./in./.degree.F., a segmental cross-sectional area of less than about 1.8 square inches and an overall length to segmental cross-sectional area ratio greater than about 0.6, establishing an electroforming zone between an anode selected from a metal and alloys thereof having a coeficient of expansion of between about 6.times.10.sup.-6 in./in./.degree.F. and about 10.times.10.sup.-6 in./in./.degree.F. and a cathode comprising the core mandrel, the cathode and the anode being separated by a bath comprising a salt solution of the metal or alloys thereof, heating the bath and the cathode to a temperature sufficient to expand the cross-sectional area of the mandrel, applying a ramp current across the cathode and the anode to electroform a coating of the metal on the core mandrel, the coating having a thickness at least about 30 Angstroms and stress-strain hysteresis of at least about 0.00015 in./in., rapidly applying a cooling fluid to the exposed surface of the coating to cool the coating prior to any significant cooling and contracting of the core mandrel whereby a stress of between about 40,000 p.s.i. and about 80,000 p.s.i. are imparted to the cooled coating to permanently deform the coating and to render the length of the inner perimeter of the coating incapable of contracting to less than about 0.04 percent greater than the length of the outer perimeter of the core mandrel after the core mandrel is cooled and contracted, cooling and contracting the core mandrel, and removing the coating from the core mandrel.
Any suitable metal capable of being deposited by electroforming and having a coefficient of expansion of between about 6.times.10.sup.-6 in/in/.degree.F. and about 10.times.10.sup.-6 in/in/.degree.F. may be used in the process of this invention. Preferably, the electroformed metal has a ductility of at least about 8 percent elongation. Typical metals that may be electroformed include, nickel, copper, cobalt, iron, gold, silver, platinum, lead, and the like, and alloys thereof.
The core mandrel should be solid and of large mass or, in a less preferred embodiment, hollow with means to heat the interior to prevent cooling of the mandrel while the deposited coating is cooled. Thus, the mandrel has high heat capacity, preferably in the range from about 3 to about 4 times the specific heat of the electroformed article material. This determines the relative amount of heat energy contained in the electroformed article compared to that in the core mandrel. Further, the core mandrel should exhibit low thermal conductivity to maximize the difference in temperature (Delta T) between the electroformed article and the core mandrel during rapid cooling of the electroformed article to prevent any significant cooling and contraction of the core mandrel. In addition, a large difference in temperature between the temperature of the cooling bath and the temperature of the coating and mandrel maximizes the permanent deformation due to the stress-strain hysteresis effect. A high thermal coefficient of expansion is also desirable in a core mandrel to optimize permanent deformation due to the stress-strain hysteresis effect. Although an aluminum core mandrel is characterized by a high thermal coefficient of expansion, it exhibits high thermal conductivity and low heat capacity which are less effective for optimum permanent deformation due to the stress-strain hysteresis effect. Typical mandrels include stainless steel, iron plated with chromium or nickel, nickel, titanium, aluminum plated with chromium or nickel, titanium pallidium alloys, inconel 600, Invar and the like. The outer surface of the mandrel should be passive, i.e. abhesive, relative to the metal that is electrodeposited to prevent adhesion during electroforming. The cross-sectional configuration of the mandrel may be of any suitable shape. Typical shapes include circles, ovals, regular and irregular polygons such as triangles, squares, hexagons, octagons, rectangles and the like. For mandrels have a convex polygon cross-sectional shape, the distance across adjacent peaks of the cross-sectional shape is preferably at least twice the depth of the valley between the peaks (depth of the valley being the shortest distance from an imaginary line connecting the peaks to the bottom of the valley) to facilitate removal of the electroformed article from the mandrel without damaging the article and to ensure uniform wall thickness. The surfaces of the mandrel should be substantially parallel to the axis of the mandrel. Thus, the core mandrel should have a taper of less than about 0.001 inch per foot along the length of the core mandrel. This is to be distinguished from a core mandrel having a sharp taper which would not normally present any difficulties in so far as removal of an electroformed article from the mandrel. This taper, of course, refers to the major surfaces of the mandrel and not to an end of the mandrel which may also be covered by an electroformed deposit. The mandrel should have a segmental cross-sectional area of less than about 1.8 square inches and an overall to segmental cross-sectional area ratio greater than about 0.6. Thus, a mandrel having a segmental cross-sectional area of about 1.8 square inches would have a length of at least about 1 inch. Excellent results have been obtained with the process of this invention with a solid cylindrical core mandrel having a segmental cross-sectional area of about 0.788 square inch (1 in. diameter) and having a length of about 24 inches.
Surprisingly, an adequate parting gap may be obtained even for electroformed articles having a small diameter or small cross-sectional area by controlling the stress-strain hysteresis characteristics of the electroformed article. For example, sufficient hysteresis alone may be utilized to achieve an adequate parting gap to remove an electroformed article from a mandrel having a diameter of about 1.5 inches in the absence of any assistance from internal stress characteristics of the electroformed article or from any difference in thermal coefficients of expansion of the electroformed article and mandrel. The internal stress of an electroformed article includes tensial stress and the compressive stress. In tensial stress, the material has a propensity to become smaller than its current size. This is believed to be due to the existence of many voids in the metal lattice of the electroformed deposit with a tendency of the deposited material to contract to fill the voids. However, if there are many extra atoms in the metal lattice instead of voids, such as metal atoms or foreign materials, there is a tendency for the electroformed material to expand and occupy a larger space.
Stress-strain hysteresis is defined as the stretched (deformed) length of a material in inches minus the original length in inches divided by the original length in inches. The stress-strain hysteresis characteristics of the electroformed article fabricated by the process of this invention should be maximized about about 0.00015 in/in.